1. Field of the Invention
The present invention relates to a method of forming a uniform graphene layer on a substrate (metal- or conductive-polymer-coated, ITO) by doping expanded graphite using various kinds of dopants (Lewis acid) to grant a positive charge thereto, dispersing the doped expanded graphite in an organic solvent using ultrasonic waves to obtain a solution in which the graphene is dispersed in the organic solvent, and electrically applying a negative voltage to the solution.
2. Discussion of Related Art
Although graphene is a material in which carbon atoms are two-dimensionally combined like graphite, unlike graphite, it is a material formed of a very thin single, double or triple layer.
It has been known that graphene is structurally and chemically stable, an excellent conductor that moves electrons about 100 times faster than silicon, and streams 100 times more currents than copper.
Such characteristics of graphene were confirmed when a method of separating graphene from graphite was discovered in 2004, and estimated characteristics were experimentally confirmed, thus stirring enthusiasm in scientists all over the world.
Graphene is formed of carbon, which is a relatively light element, to enable one or two-dimensional nano-patterns to be easily processed and characteristics of a semiconductor-conductor to be easily adjusted. Further, various chemical combinations that carbon has enable the fabrication of a wide range of function devices such as a sensor, a memory, etc.
In 2008, graphene was designated as one of the top 100 among global future technology by MIT, and currently, Korea Institute of Science & Technology Evaluation and Planning and Samsung Economic Research Institute have selected graphene-related technology as one of the top 10 among technology that will change our life within 10 years in Korea.
As previously described, despite superior electrical/mechanical/chemical properties of graphite, since a method of mass synthesis thereof has not been developed, research into actually applicable technology has been very limited.
One conventional mass synthesis technique includes mechanically pulverizing graphite to be dispersed in a solution, so that the results are formed as a thin film using a self-assembly phenomenon. This method enables synthesis at a low cost, but numerous pieces of graphene overlap, and the overlap structure causes electrical and mechanical properties thereof not to meet people's expectations.
One of the most widely used method of forming a graphene layer is “the filtration and transfer method” suggested by Lee, Jong-Hak (Adv. Mater, 2010). However, when filtering graphene in this method, a specific membrane named AAO is required, a number of processes are required, and the graphene should be transferred to a desired substrate. In this method, the graphene layer formed during a process of transferring is easily broken, and when the graphene is obtained in a powder state without transferring, a binder is required to be loaded to an electrode.
Electrodeposition from graphene oxide currently suggested by Liyun Chen (Small, 2011) has exhibited a process simpler than the transferring method, and a graphene layer directly formed on a substrate. However, an oxidation process for forming oxide may cause defects of graphene after a reduction process.
Technology for depositing graphene on a substrate and using doped graphene has been developed, as disclosed in Korean Patent Application Nos. 10-2009-0035082 (Apr. 22, 2009) and 10-2009-0035081 (Apr. 22, 2009). The disclosures and the present invention are similar to each other in that they are directed to a method of depositing graphene on a substrate using deposition, and expanded graphite and doped graphene are used to fabricate graphene. However, in the deposition method disclosed in the patent applications, deposition is performed on a substrate by spraying an aerosol, which is different from the present invention in which electrodeposition is performed. Further, doping using boron and phosphorus is performed by substituting with carbon disclosed in the above disclosures, which is different from the doping disclosed in the present invention. The disclosed patent applications are provided to deposit formed graphene on a desired substrate, not to obtain a large amount of pure graphene.
Accordingly, the present inventors have overcome the above problems, and found a method of forming graphene below by which a simple process method can be provided, graphene of a high degree of purity can be obtained, and mass production is enabled.